Vasan Sivalingam in the PhD programme Process, Energy and Automation Engineering at the University of South-Eastern Norway, Faculty of Technology, Natural Sciences and Maritime Sciences, will be defending his thesis for the degree of philosophiae doctor (PhD).
Title of dissertation: Syngas fermentation and Microbial Electrosynthesis Process Integration to Advance Biogas Production.
Norway implements several CO2 emission control measures to become a low-emission society by 2050 and to confront global climate risks. As a part of this strategy, it strives to be the first nation to end the sales of fossil-fuelled vehicles by 2025. Passenger cars are being replaced by electric vehicles. However, heavy long-distance transport still relies on fossil fuels that can be substantially replaced by liquified biogas. The biogas demand is more extensive than the current supply volume. The feedstock availability and lower methane yield are the primary challenges for the complete substitution of biogas as a clean fuel. This project investigates syngas (mixture of H2, CO2 & CO) fermentation and microbial electrosynthesis (MES) as sustainable technologies to advance biogas yield and methane content to add value to the national goal against climate risks.
1 to 25 bar elevated H2 headspace pressure was tested to improve the gas-liquid (GL) mass transfer. The 15 bar was identified as the optimum pressure, which enhanced the gas uptake rate by 250 % and product synthesis (acetate synthesis) by 81 %. Then moving bed biofilm (MBB) was integrated into the 15-bar pressure condition as a strategy to overcome the kinetic growth limitation of the microbes who account for the fermentation. Therein, the gas uptake rate improved by 33 %, while product synthesis increased by 48 %. Further, the impact of MES integration on syngas fermentation was examined by supplying an electric potential difference to biofilm on electrodes. This study was performed in two different syngas compositions. In the first study, H2 was the only gaseous substrate, while CO2 was in the liquid medium as bicarbonate. In study two, an industrially relevant syngas mixture (15 % CO, 15 % H2, 20 % N2 and 50 % CO2) was used as the only substrate. Both studies set the benchmark potentials for bio electrochemically mediated syngas fermentation: respectively – 175 mV, and – 150 mV vs. Ag/AgCl (3.0 M NaCl) reference electrode. These are the lowest potential values among the other similar studies. With the help of an electric potential, more acetic acid (promotor to biomethane) could be produced. Moreover, these MES studies revealed that water, ammonium, and organics are the source of additional reducing equivalents that improve the fermentation process.
The results collectively demonstrated that elevated syngas pressure, MBB, and MES incorporation improved acetate synthesis, which further converted into methane. Thus, syngas fermentation and MES process integration advance the biogas production.